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Parallelism, Flatness and Plus/Minus

Parallelism, Flatness and Plus/Minus

Parallelism, Flatness and Plus/Minus

(OP)
I have a surface on a part that I am dimensioning w.r.t to a surface Datum A. If I put a dimension X +/- .001, is there no point in putting a // to Datum A of .002/.003? Because the plane has to lie within that tolerance from the +/- .001 anyways correct? Also for the same surface, if I callout // of .004 w.r.t Datum A, that is sort of like saying the flatness of the surface is also .004 right? Seems like when you get to tight tolerances flatness, +/- and //ism go hand in hand and sort of say the same thing.. Thanks in advance

RE: Parallelism, Flatness and Plus/Minus

Rule 1/Taylor principle does indeed apply.

This has been discussed before, I believe it was recently touched on here thread1103-383476: Straightness tolerance on a piece of sheetmetal & earlier thread1103-342370: Application of Rule #1 (Taylor Principle)

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RE: Parallelism, Flatness and Plus/Minus

Yes, in general flatness is a refinement of parallelism and parallelism may be refinement of size if envelope requirement applies.

Note that parallelism controls surface in question WRT the surface marked as A, but it doesn't control surface A itself.

Rule 1 tells you even less, ass both surfaces are allowed to vary as they please.

If you want both surfaces to be flat and parallel, it makes sense to use flatness on your A surface.

Also, as you imply Rule 1, I imply you operate to ASME standards, but if it's not the case, the situation in ISO is slightly different.

All together can be summarized in the enclosed picture - note that surface A has the worse condition than surface being controlled.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

RE: Parallelism, Flatness and Plus/Minus

As a side note, I've always found it interesting that if you read Rule #1, it only mentions form. So a purist might say that Rule #1 itself says nothing about parallelism (which is not form, but orientation). Obviously parallelism is a by-product of Rule #1, so I'm not saying anything's wrong -- I certainly agree with the other comments here. I'm just voicing a thought that I've noticed.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

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